Advantages and Technical Aspects of Digital Television
Advantage Digital Television
1) Increased Noise Immunity
In the case of an analog signal, noise adds to the signal, degrading it as an indivisible whole. In a digital signal, while the noise does not exceed the level causing misinterpretation of the data, it can be extracted without affecting the signal.
2) Increased Capacity for Manipulation
The increasing number of available algorithms can generate effects, expand creative possibilities, and improve the quality of signal processing.
3) Bandwidth Control
Once digitized, the signal can be compressed, allowing for the placement of several signals in the same bandwidth occupied by one analog signal. The only aspects that remain in the analog domain are the capture and presentation.
Overview of Digital Terrestrial Television Transmission
- Bandwidth Administration: Through compression, redundant information can be eliminated from the data frame, occupying less bandwidth.
- Increased Noise Immunity During Transmission: With the help of algorithms, errors can be corrected, and within a certain range, some misinterpreted data can be recovered.
- Separate Transmission of Video Signal Components: To avoid signal degradation, the components of the digital video signal (luminance and color differences) are digitized and transmitted separately to the receiver.
- Possibility of Increased Horizontal Resolution: For example, 720 * 480 in NTSC or 720 * 576 in PAL, compared to 350 lines for an analog TV signal.
- Possibilities of Progressive Scan: For correct reproduction, 24 or more frames per second should be transmitted. To eliminate flicker, interlaced scanning requires 48 or more frames per second. With interlaced scanning, 25 or 30 frames per second are transmitted, but at a frequency of 50 or 60 fields per second, which the human eye cannot perceive. Digital transmission eliminates the constraints of interlaced scanning.
- Increased Vertical Color Resolution: The alternating lines characteristic of composite video signals in PAL systems reduce the vertical resolution of color. By handling the signal in the digital domain, the need for composite video disappears, eliminating this drawback.
- Possibility of Using Different Aspect Ratios (16:9): A wider horizontal field of vision is more pleasing to the eye. Presenting an image 30% wider than the current 4:3 screen offers a significant improvement.
- Digital Audio (CD Quality): Audio can be transmitted with a bit rate ranging from 128 to 256 kbps, compared to 4-6 Mbps for CD quality.
- Multiple Audio Channels: The availability of higher bit rates, combined with techniques like Dolby AC-3, allows for the handling of multiple audio channels, enabling viewers to enjoy surround sound like in movies.
- Data Transmission and Other Services: The transmission channel becomes a broadband data channel. Thanks to video and audio compression, the remaining transmission capacity can be used for multiple applications.
- Possibility of HDTV: The large capacity of the channel bit rate offers the possibility of transmitting high-definition signals. Depending on the format used, this requires between 14 and 18 Mbps.
- Quantization Noise: If an 8-bit word is used, the possible excursion range of the video signal is divided into 256 levels. Quantization defects become noticeable on image surfaces with smooth gradients of variation or color level.
- Compression Defects: Compression algorithms predict the content of subsequent pictures and only transmit the difference information. This can lead to artifacts in the decoded image.
Parts of the Transmission System
Analog to Digital Conversion
This is only necessary if the program output is delivered in analog form.
Compression
MPEG-2 is used for video compression, with the following features:
- 8-bit word size
- Manages video resolutions up to 720 * 576 for PAL and 720 * 480 for NTSC
- 4:2:0 component video sampling
- Can work with interlaced or progressive scanning
- Can handle bit rates of up to 15 Mbps
- Uses the DCT (Discrete Cosine Transform) technique to convert spatial domain elements to the frequency domain, allowing for redundancy removal
- VLC techniques reduce the bit rate
- Motion compensation reduces temporal redundancy
- Group of Pictures (GOP) structure
For audio, MPEG-1 Layer 2 and Dolby AC-3 are used.
Multiplexer
Each channel is compressed by an encoder, which delivers a program stream. The multiplexer combines the different channel streams into a transport stream.
Modulator
The transport stream provided by the multiplexer is modulated onto the carrier frequency of the channel.
Transmitter
The carrier signal is amplified to appropriate levels for proper coverage.
Antenna
An antenna system filters out spurious signals and ensures that the transmitted bandwidth complies with relevant standards.
Home Receiver Parts
Antenna
This can be a fixed or portable Yagi antenna.
Receiver/Decoder
This unit tunes to the channel, demodulates the incoming RF signal, and delivers the transport stream to the decoder. The decoder decompresses the signal and performs digital-to-analog conversion to deliver video and audio signals to the screen. There are two types of receivers: integrated receivers, which include the display, and set-top boxes, which only receive and decode the signal. The most common outputs are Y, Pb, Pr for video and a stereo pair for audio.
Display
Set-top boxes allow for the use of new display technologies like plasma screens.
Digital Television Standards
- ATSC: Advanced Television Systems Committee (USA)
- DVB-T: Digital Video Broadcasting – Terrestrial (Europe). DVB-S is for satellite and DVB-C is for cable.
- ISDB-T: Integrated Services Digital Broadcasting – Terrestrial (Japan)
ATSC
System Characteristics
- 6 MHz channel bandwidth
- Provides two modulation subsystems: 8-VSB for terrestrial broadcasting over the air (offers a net bit rate of 19.3 Mbps) and 16-VSB for cable transmission (bit rate of 38.6 Mbps)
Variety of Video Program Combinations
- 1 High Definition (HD) channel (18 Mbps)
- 1 HD channel plus one Standard Definition (SD) channel (14 Mbps and 4 Mbps respectively)
- 3 SD channels (6 Mbps each)
- 4 SD channels (4.5 Mbps each)
- 6 SD channels (3 Mbps each)
Video compression is based on the MPEG-2 main profile, including I, P, and B frames; 4:2:0 video component sampling; and 8-bit precision. For SD variants, MP@ML (main profile at main level) is used, while HD variants correspond to MP@HL.
For audio, the ATSC standard specifies the Dolby AC-3 standard, comprising 1, 2, 3, 4, 5.1 channels (actually 6 channels), with an optional 0.1 LFE (Low Frequency Effects) channel with a very limited bandwidth.
Audio Services
The ATSC standard offers different types of audio services:
- Complete Main audio service (CM)
- Main audio service, music and effects (ME)
- Associated services: Visually Impaired (VI), Hearing Impaired (HI), Dialogue (D), Commentary (C), Emergency Message (E), Voice-Over (VO)
Language alternatives can be communicated efficiently in two stereo channels or one mono channel. Another possibility is to transmit a main program and associated ME and D channels for each language.
The AC-3 standard works with a minimum word size of 16 bits, which can reach up to 24 bits, with a sampling frequency of 48 kHz. It supports bit rates from 32 kbps to 640 kbps.
Receiver Characteristics
- Prepared to receive a primary service of 384 Mbps
- Downmixing: Allows a single multi-channel transmission to be decoded according to the capabilities of each receiver. The downmix is not predefined in the encoder and is fully compliant with Dolby standards. A single multichannel frame is transmitted, and decoders can extract 5.1 channels, 4-channel Pro Logic, 2-channel stereo, or a single mono channel.
- Loudness Uniformity: Designed to avoid unwanted variations in average loudness level.
- Dynamic Range Control: The level of dialogue is not altered, loud sounds are attenuated, and weak sounds are amplified.
Transmission Process
The transport stream is composed of packets of 188 bytes (187 data bytes and 1 synchronization byte). Each data frame is composed of two data fields of 313 segments. Each segment lasts 24.2 ms, and each data field lasts 77.3 ms. Each segment contains 832 symbols (828 data, 4 synchronization, and FEC). Each symbol represents one of 8 possible levels (3 bits), which are then amplitude modulated onto the vestigial sideband carrier.
The process begins with randomization, followed by FEC using Reed-Solomon encoding, which adds 20 parity bytes to each 187-byte packet, resulting in a total of 207 bytes. The convolutional interleaving process is a 52-segment interleaver, defining a depth of 1/6 of the data field. The Trellis encoder uses a rate of 2/3, consisting of 12 coders working sequentially. The bit pattern provided by the Trellis encoder is mapped to 8 levels per symbol, with 3 bits per symbol.
DVB
System Features
- Designed for an 8 MHz channel bandwidth
- Uses COFDM (Coded Orthogonal Frequency Division Multiplexing) modulation scheme
- Accepts two fundamental modes: 2k (1705 carriers, used for operations with a single transmitter and small SFN networks with limited transmitter power) and 8k (6817 carriers)
- In both cases, the carriers can be modulated by QPSK or QAM
- Picture and sound encoding are based on the ISO/IEC MPEG-2 standard
- Can transmit programs of 625 lines with studio quality and aspect ratios of 4:3, 16:9, or 20:9, requiring the system to operate at up to 9 Mbps
Audio
DVB uses the MPEG Layer II (MUSICAM) standard for audio, which can handle mono, stereo, or surround sound in multiple languages. Dolby AC-3 is also included.
Transmission Process
The MPEG-2 transport stream provided by the multiplexer is composed of packets of 188 bytes (1 synchronization byte and 187 data bytes). The outer coding process adds redundancy using Reed-Solomon encoding at the packet level (RS(204,188, t=8)). Each 188-byte packet is randomized and 16 parity bytes are added. The inner coding process adds additional redundancy, which can be configured to 1/2, 2/3, 3/4, 5/6, or 7/8, with 1/2 providing the greatest robustness. One or two input frames are interleaved bitwise, dividing them into 2, 4, or 6 sub-frames depending on the type of modulation. The block interleaving process is repeated twice during each OFDM symbol in 2k mode and 48 times per symbol in 8k mode. In hierarchical transmission, QPSK, 16-QAM, or 64-QAM modulation can be used. For hierarchical = 1, only one modulation type can be used, while for hierarchical = 2 or 4, different modulation types can be used for different groups of carriers.
Reference Signals
There are three types of reference signals: scattered pilot (1 cell), continuous pilot (2 carriers), and TPS (Transmission Parameter Signaling) (2 carriers).
TPS Information
The 68 TPS information bits are allocated as follows: 31 bits for startup, 16 bits for synchronization, 7 bits for information, and 14 bits for redundancy.
COFDM Parameters
COFDM provides the user with certain parameters:
- Mode: 8k, 2k
- Type of Modulation: QPSK, 16-QAM, 64-QAM
- Modulation parameters: 1 (non-hierarchical), 2 and 4 (hierarchical)
- Inner code protection: 1/2, 2/3, 3/4, 5/6, 7/8
- Guard interval length: 1/4, 1/8, 1/16, 1/32
These parameters allow for greater robustness in the transmission.
ISDB-T
System Characteristics
- Uses OFDM modulation technology
- 6 MHz channel bandwidth (can also use 8 MHz)
- MPEG-2 encoding and multiplexing
Parameters
- Mode: 2k, 4k, 8k
- Number of carriers: 1405 (2k), 2809 (4k) for mobile phones, 5617 (8k)
- Modulation type: QPSK, DPSK, 16-QAM, 64-QAM
- Number of segments: Less than or equal to 13
- Outer code: RS(204,188)
- Inner code: 1/2, 2/3, 3/4, 5/6, 7/8
- Guard interval: 1/4, 1/8, 1/16, 1/32
Services
- 1 HDTV service + 1 audio + 1 data
- 1 service for mobile reception of SDTV + 1 audio for SDTV receivers + 1 data
- 3 SDTV services for fixed receivers + 1 data service
Technical Aspects
1) Sensitivity and C/N
Sensitivity is a receiver parameter that determines the minimum received power required for operation. The input signal power must exceed the minimum value to be amplified and not exceed the maximum value to avoid amplifier saturation. There are two thresholds: quality (problems appear in the image around 2 dB above the cutoff) and cutoff (reception is completely lost).
2) Scope and Coverage
This refers to the geographical area that can be reached by the transmitted signal.
3) Fixed and Mobile Reception
For fixed reception, directional antennas are used. The 8-VSB system does not work well in mobile conditions, while COFDM does (in 2k mode). Both COFDM 2k and 8-VSB significantly sacrifice bit rate for mobile reception.
4) Performance in the Presence of Ghosts and Multipath
Digital television systems are designed to mitigate the effects of ghosts and multipath interference.
5) Immunity from Impulsive Noise
Impulsive noise, produced by electric motors, affects transmission and consists of short bursts. Digital television systems are more resistant to impulsive noise than analog systems.
6) Adjacent Channel Interference
Unlike analog television, digital television does not require a clear channel between adjacent channels. There are two types of interference: analog adjacent channel on digital channel (seen as noise by the digital signal, deteriorating the BER) and digital adjacent channel on analog channel (causes a pattern of white dots in the analog image).
7) Co-Channel Interference
There are three types of co-channel interference: analog on digital, digital on analog, and digital on digital. Digital on digital interference acts as noise on the main carrier, directly affecting the C/N.
8) Transmitter Power
Determined by the desired scope or coverage area.
9) Margins and Spectrum Planning
Spectrum planning must take into account various factors, including:
- Changes in atmospheric and geographic conditions
- Seasonal changes in propagation conditions
- Adjacent channel and co-channel interference
- Gaussian and impulsive noise
10) Useful Bit Rate
The higher the bit rate, the greater the number of broadcasting services and the better the quality. The bit rate depends on the modulation scheme employed.
11) High Definition Video
Digital television enables the transmission of high-definition video signals.
12) Multichannel Audio
Digital television supports stereo programs (2 channels), an SAP channel, and multi-channel surround sound offered by Dolby AC-3 and MPEG.
13) Relationship Between Bit Rate and Robustness
There is a trade-off between bit rate and robustness. Higher bit rates allow for more services and better quality but are more susceptible to noise and interference.
Economic Aspects
Social and Cultural Good
Television is considered a social and cultural good, providing information and entertainment to the public.
Simultaneous Transmission (Simulcast)
This involves transmitting new digital services simultaneously with existing analog services during a transition period.
Economy of Scale
Digital television benefits from economies of scale, as the cost per channel decreases with the number of channels transmitted.
Single Frequency Network (SFN) and Gap Fillers
SFNs allow multiple transmitters to broadcast the same signal on the same frequency, improving coverage and efficiency. Gap fillers are low-power transmitters used to fill in coverage gaps in areas with difficult reception.
Cost of Receiver Replacement
There are two types of receivers: integrated receivers and set-top boxes. Integrated receivers include the display and are more expensive, while set-top boxes only decode the signal and are cheaper.
Cost of High Definition
HD receivers are more expensive than SD receivers, limiting access for some consumers.
Business Model
Digital television offers new business opportunities, including:
- Multiple channels
- High definition services
- Transmission of broadband data
- Mobile reception